When the opponents open the bidding the next player has several options:

1. Pass

2. Overcall in a suit

3. Overcall in no-trumps

4. Double

5. Use a convention to show a special sort of hand

Overcalls usually show a 5 card suit, a jump overcall should show a 6 card suit, and a double jump overcall is a pre-emptive 7 card suit. Overcalling at the 4 level in a major is largely pre-emptive , minimum 7 card suit but with decent opening points

To overcall or double? In general ,unless you are very strong, overcall with a 5 card major and double with a 4 card major if the hand shape is suitable. With a very strong hand and a good 5 card suit (17+ pts) it is better to double first then bid your suit if partner does not respond in it.

Before overcalling determine the purpose of the bid. Are you expecting to win the contract? Or make a sacrifice? Or are you just trying to push the opponents up? Or is it lead directional?

Remember that partner will normally lead your overcall suit, so making overcalls on poor suits when you know the contract belongs to the opponents, probable helps declarer more than your side. Apply the suit quality test (SQT)- add the number of cards in the suit (normally 5) to the number of honours in the suit, and at the 1 level the suit quality should be 7 or more and at the 2 level 8 or more . This is especially important if your side is vulnerable and opponents are not. If partner has already passed, and you are vulnerable and opponents are not, it is important to apply the SQT , before bidding. Especially if it is probable that the opponents have the majority of high cards

How many points do you need for an overcall? The range can be 8 to 18 points, for a suit bid, though rarely above 15 because you usually have other options. These days doubles of opponents suit bids are for take-out up to at least 2 spades so with higher point hands this may be a better option than a simple overcall. Vulnerability is an important consideration. Making poor overcalls when your side is vulnerable and the opponents are not can lead to some large adverse penalties! Jump overcalls when you are vulnerable should be a 6 card suit and 12-16 pts, but weaker if you have a 7 card suit to 2 honour cards. Non vulnerable jump overcalls are usually in the 8-11pts range.

How do show a strong hand with a good suit. Double first then bid your suit if partner does not choose it.

An suit overcall usually shows a single suit headed by honour cards. At the
one level it may be occasionally a 4 cards suit.. After two passes (i.e. 4th
in hand) the overcall may be“balancing”,
in case partner in second seat had points but was not able to bid. In this case
“borrow “
a king to see if you have enough points to make a bid. The number of spades
you hold is an important factor. It is frequently better to leave opponents
in a minor suit than overcall and then be outbid. This is similar to the position
where no-one has bid and you wish to open 4th in hand, and you are
uncertain whether to bid, apply the rule of 15 (add points to the number of
spades, and if this is less than 15 it is usually better to pass)

A 1 no-trump overcall should show 15-16 pts (or 15-17 ), with at least a stop in the opponents suit

Overcalling opponents 1NT opening bid is based more on shape than points but usually a minimum of 9 pts is required. There are many conventional bids to show hands that contain 2 or more biddable suits. ASTRO is a popular convention. This is able to show a 5 card major and another 4 card suit.

If partner makes a suit overcall, as a priority raise the suit if you can, but if you change suit, you should have either a very good suit of your own, or a suit plus secondary support for partner’s overcall suit (i.e 3 cards). Bid the opponents suit to show a strong hand (Unassuming Cue Bid) with good support for partners suit, to differentiate between pre-emptive support for partner’s overcall and a good value raise in the suit.

Defence to opponents overcalls. The so -called negative double with a minimum point count of 8 is the most powerful weapon in your armoury, assuming you cannot raise partner’s suit. This usually shows the other two suits, but is totally flexible.